The invention relates generally to spray guns for charging and distributing powders, such as electrostatically-charged powder paint particles, for deposition on the surface of a workpiece.
Conventional powder applicators are based on exterior electrostatic charging of a dispersed particle cloud as partly described in U.S. Pat. No. 5,711,489. This patent also describes means for improving the particle dispersion by a rotating airstream in the interior of the gun, as well as temperature and humidity control of the powder feeding airstream. Other conventional powder applicators are based on rotating bell cup principles such as described in U.S. Pat. No. 5,353,995.
U.S. Pat. No. 6,254,684 describes an internally charged powder spraying applicator wherein the powder is pre-charged in the interior charging chamber of the gun. The process of interior charging requires interior high voltage electrodes and at least one ground electrode. The '684 patent discloses a first design in which a round powder cloud pattern is produced by means of a round conical deflector and a second approach wherein a flat spray pattern is generated by means of a slotted nozzle. Generation of a rounded powder cloud is important in cases where a robot or some other reciprocating machine is used to move an applicator around or inside of the painted workpiece object. The cloud generator in the '684 patent has some disadvantages regarding contamination of the deflector by paint particles which leads to coating defects on the workpiece due to dripping of powder agglomerates on the surface of the workpiece. Generation of a flat spray pattern is less subject to contamination and is more widely used for flat workpiece surfaces. However, a flat pattern is more difficult to use for curved workpiece surfaces and for robotic applications, in that this design approach requires more robot arm reorientations when programming robot strokes for effecting desired surface covering.
U.S. Pat. No. 6,053,420 discloses a conical powder dispersing unit based on a tangential air/powder mixture flow which provides a round powder cloud spray pattern, yet avoids use of a deflector in the direction of the powder flow. While this approach provided an improvement to U.S. Pat. No. 5,711,489, it has nevertheless been limited to cone sizes of 50 to 170 mm. diameter which is rather large for robotic applications. Additionally, at this size, the powder cloud becomes rather “soft” in order to be moved by a robot arm. The approach disclosed in the '420 patent additionally anticipated a direct feeding from a fluidized powder bed feeder in a dense powder flow directly through a relatively small orifice.
German Published Patent Application No. 19614193 describes the combination of interior or exterior powder charging combined with exterior tangential swirl flow which is intended to produce a softer rotating round pattern powder cloud while avoiding use of deflectors in the powder stream.
There is seen, therefore, to be a need in the art for a powder applicator with the capability for utilizing shaping air rather than deflectors, yet have the capability to maintain the powder/air mixture in a more intense motion.